This invention relates to electrolytic storage cells that utilize a solid electrolyte, and, more particularly, to the initial wetting of the electrode materials to the electrolyte.
Rechargeable storage cells are electrochemical devices for storing and retaining an electrical charge and later delivering that charge as useful power. A number of such storage cells are typically connected together electrically to form a battery having specific voltage or current delivery capability. Familiar examples of the rechargeable storage cell are the lead-acid storage cell used in automobiles and the nickel-cadmium storage cell used in portable electronic devices such as cameras. Another type of storage cell having a greater storage capacity for its weight is the nickel oxide pressurized hydrogen cell, an important type of which is commonly called the nickel-hydrogen cell and is used in spacecraft applications.
Yet another type of storage cell is the sodium-sulfur storage cell, which has been under development for over 20 years for use in a variety of terrestrial applications such as nonpolluting electric vehicles. The sodium-sulfur storage cell has the particular advantage that its storage capacity per unit weight of storage cell is several times the storage capacity of the nickel-hydrogen cell. The sodium-sulfur storage cell therefore is an attractive candidate for use in spacecraft applications as well as automotive applications.
In general, the sodium-sulfur electrolytic storage cell has an outer housing and a piece of an alumina-based ceramic within the outer housing. Sodium is placed into a first chamber defined on one side of the ceramic, and sulfur is placed into a second chamber on the other side of the ceramic. The storage cell is heated to a temperature of about 350.degree. C., at which temperature both the sodium and the sulfur are molten. The liquid sodium is the anode of the storage cell, the liquid sulfur is the cathode, and the solid ceramic is the electrolyte. Electrical energy is released when sodium ions diffuse through the ceramic into the sulfur, thereby forming sodium polysulfides. Electrical energy can be stored when the process is reversed during charging of the battery, with an applied voltage causing the sodium polysulfides to decompose to yield sodium and sulfur, and the sodium ions diffuse through the ceramic electrolyte back into the first chamber.
One configuration of the sodium-sulfur cell is known as the "planar" cell. The term "planar" is used in this context to mean that the ceramic electrolyte is generally planar. Other configurations such as tubular forms having a cylindrical tubular electrolyte are also known. The planar design has the advantage that the active area of electrolyte is relatively larger per unit weight of cell than for a cylindrical design. Planar sodium-sulfur cells are disclosed in U.S. Pat. Nos. 3,765,945 and 3,789,024, for example.
One of the problems encountered in the use of sodium-sulfur cells, and particularly in relation to planar sodium-sulfur cells, is achieving initial wetting of the liquid sodium to the beta-double prime aluminum oxide electrolyte. The wetting angle of sodium to aluminum oxide is only about 90 degrees, and therefore the sodium does not easily wet and spread on the aluminum oxide. If wetting is not achieved, there may be start -up inconsistencies and uneven current densities at start-up of the cell. These effects may result in unsatisfactory performance of the electrolytic cell over extended periods of operation.
There have been proposed several techniques for improving the initial wetting of the electrolyte by liquid sodium. In one, lead is deposited upon the electrolyte surface by autoabrasion. The use of the lead deposition approach can result in contamination of the electrolyte, particularly along its grain boundaries, leading to premature failure of the electrolyte during service.
In another approach, the electrolyte is exposed to lead acetate solution by coating the electrolyte with an aqueous solution of the lead acetate. Lead acetate deposited upon the electrolyte aids in achieving initial wetting when the lead acetate at the surface is reduced to lead in contact with the molten sodium of the cell. However, the aluminum oxide is sensitive to water, and therefore a drying step is required. The lead can be dissolved into the sodium during service, leading to long-term sodium-ion displacement in the aluminum oxide. This can result in localized strain within the electrolyte, leading to its premature failure. The lead acetate approach may also be unacceptable because of its slow reaction rate.
There is therefore a need for an improved approach to achieving wetting of the electrolyte by the electrode material, and specifically improving the wettabllity of aluminum oxide by sodium. The present invention fulfills this need, and further provides related advantages.